The present invention relates to a device for monitoring force and pressure in injection moulding machines, with at least one sensor for measuring the deformation of a machine part that is deformed by the closing or injection pressure.
It is typical for injection moulding machines that both in the closing procedure and the injection procedure, forces of different magnitudes occur and have to be monitored.
The closing force with which the mould halves are held together while plastic material is filled into the hollow space in the mould, for example, is typically in the order of 1,000 kN. On the other hand, if an object accidentally remains between the opened mould halves, a force of a few N occurring when this object is compressed should be sufficient to actuate a mould protection device and to terminate the closing procedure.
On the injection side, two different procedures take place that can also be best controlled by monitoring the forces and pressures occurring. Firstly, the granulate material supplied to a screw is plasticised and conveyed into the space in front of the screw. The screw gradually moves backwards under the influence of the back pressure in the plastics material. This back pressure can be, for example, 40 bar. Once the desired amount of plastics is plasticised, the rotation of the screw is ended and it is used functioning as a piston to inject the plastics material. The pressure occurring then can easily be 2,000 bar.
Particularly in the case of toggle machines and electrically actuated machines, monitoring of force takes place in that the expansion, compression or bending of a machine part subjected to the forces is monitored, for example by means of expansion measuring strips. Such devices do not, however, function over the whole range of forces occurring. Thus, if forces are to be monitored in the mould protection range on the closing side and the back pressure range on the injection side, it must be ensured that the sensors suitable for this are not overloaded in the closing pressure or injection pressure range. According to the invention, a supporting body is provided for this, that relieves the machine part monitored by the sensor as soon as the closing or injection pressure exceeds a certain value that is less than half its maximum value.
For the force range in which the sensor is bypassed, a further sensor can be provided that monitors the closing and injection pressure up to its maximum value.